PROJECT SUMMARY Immune cells serve a critical role after myocardial infarction (MI) and are necessary for proper wound healing and tissue repair, where either an impaired inflammatory response or excessive inflammation leads to defects to recovery of function. Properly balancing this process and understanding the mechanisms which regulate this process thus have great therapeutic value. We recently discovered that the BMP antagonist Grem2 regulates inflammatory cell recruitment in the heart after experimental MI. In addition, our laboratory previously published data showing that Grem2 regulated the levels of circulating leukocytes after myocardial infarction, suggesting an important, uncharacterized role in the inflammatory response. My preliminary data here also suggest that Grem2 serves an important role in regulating hemopoiesis in bone marrow BM hemopoietic stem and progenitor cells (HSPCs) - the cell population known to activate in response to injury. My findings show that loss of Grem2 during homeostasis leads to shifts in the BM HSPC population that resemble changes after MI, and concurrent loss of Grem2 after MI leads to even more severe changes. Additionally, this shift is associated with a myeloid bias present in BM clonogenic progenitors and peripheral blood (PB) leukocytes. Moreover, patients who possess the GREM2 Q76E variant ? a mutation that enhances the proteins inhibitor activity ? demonstrate perturbed hemopoiesis reflected in increased hemopoietic disease risks and shifted CBC lab values opposite to those observed in our Grem2-/- mice. As this posits a novel role for Grem2 in regulating BM hemopoiesis both during homeostasis and in response to MI, this proposal will address this knowledge gap by deepening our understanding of Grem2's role in hemopoiesis. To do so, specific multipotent, oligopotent, and committed precursor cell populations will be assayed in WT and Grem2-/- BM to identify populations regulated by Grem2 function. Moreover, absolute cell counts will be measured in order to assess for changes in BM cellularity, and proliferative capacity of populations that exhibit differences will be measured. These studies will be supplemented with functional assays using colony forming assays to measure biases in differentiation potential of progenitor populations upstream of shifted BM cell populations. Detection of aberrant BMP signaling in these populations along with administration of DMH1, a BMP inhibitor, to reverse phenotypes will be carried out to determine that consequences arise from defective BMP antagonism. Regulation of HSPC mobilization by Grem2 will also be explored. Paralleling this, I plan to use genetics and informatics tools to characterize clinical consequences of altered GREM2 function in patients, focusing on relevant hematopoietic disease risks and cardiovascular outcomes. The proposed work will provide important new information regarding the role of BMP signaling inhibition by Grem2 in BM hemopoiesis both during homeostasis and in response to MI. This knowledge may eventually lead to new strategies to treat human diseases that are caused by impaired inhibition of BMP signaling.